1. Field of the Invention
The present invention relates to a variable attenuator, a composite variable attenuator and mobile communication apparatus.
2. Description of the Related Art
Generally, in mobile communication apparatus such as mobile telephones, variable attenuators have been used to variably attenuate high frequency signals by using switches to select among a plurality of attenuators having different attenuation values.
FIG. 8 shows a prior art variable attenuator for use in a microwave band. A variable attenuator 70 includes an input terminal 71, an output terminal 72, field effect transistors (FET) 731 to 733 and 741 to 743 for switching conduction and cutoff between input and output, and T-type resistance attenuators 751 to 753, each having losses of A (dB), B (dB) and C (dB), respectively. In this configuration, each of the drain electrodes D of the FETs 731 to 733, which work as switches at the input end, is connected to the input terminal 71 via a capacitor C71, while each of the drain electrodes D of the FETs 741 to 743, which work as switches at the output end, is connected to the output terminal 72 via a capacitor C72. Also, the source electrodes S of the FETs 731 to 733 are connected to one end of respective resistors R71 to R73 of the T-type resistance attenuators 751 to 753 via capacitors C73 to C75, respectively; while the source electrodes S of the FETs 741 to 743 are connected to one end of respective resistors R74 to R76 of the T-type resistance attenuators 751 to 753 via capacitors C76 to C78, respectively. Further, the other ends of the resistors R71 to R73 of the T-type resistance attenuators 751 to 753, respectively, are connected to the other ends of the resistors R74 to R76, respectively, to connect their nodes to a ground via resistors R77 to R79, respectively. Further, the gate electrodes G of the FETs 731 to 733 and 741 to 743 are connected to the ground via capacitors C79 to C81 and C82 to C84, respectively, and are connected to control terminals Vc71 to Vc73 and Vc74 to Vc76, respectively, via inductors L71 to L73 and L74 to L76, respectively, for cutting-off high frequencies.
A negative voltage at the same level as the pinch-off voltage of the respective FET to be controlled or 0 V is selectively applied to each of the control terminals Vc71 to Vc76: If 0 V is applied to the control terminals Vc71 and Vc74 in the first route and a negative voltage at the same level as the pinch-off voltage of the FETs 732, 742, 733 and 743 to be controlled is applied to the control terminals Vc72, Vc75, Vc73, and Vc76 in the second and third routes, respectively, the channel resistance between the drain and the source of the FETs 731 and 741 becomes sufficiently lower than the characteristic impedance of the T-type resistance attenuator 751. On the other hand, the channel resistances between the drains and the sources of the FETs 732, 742, 733 and 743 becomes extremely high due to expansion of depletion layers within the channels. As a result, microwaves input from the input terminal 71 pass through only the first route including the T-type resistance attenuator 751, while the second and third routes including the T-type resistance attenuators 752 and 753, respectively, are disabled. Accordingly, attenuation between the input terminal 71 and the output terminal 72 becomes A (dB).
To switch the attenuation between the input terminal 71 and the output terminal 72 to B (dB), 0 V is applied to the control terminals Vc72 and Vc75 in the second route and a negative voltage at the same level as the pinch-off voltage of the FETs 731, 741, 733 and 743 to be controlled is applied to the control terminals Vc71 and Vc74 in the first route, and Vc73 and Vc76 in the third route, to enable only the second route including the T-type resistance attenuator 752. Switching the attenuation to C (dB) is also achieved by a similar operation to the above. The above operations allow variable control of a plurality of attenuations, but discontinuously.
However, the conventional variable attenuator described above has a problem in that the attenuation can not be variably controlled in a continuous manner due its configuration in which it uses switches to select among a plurality of attenuators having different attenuation values.
Also, it tends to require many component parts because the number of FETs that compose a switch in each channel is a number that is a multiple of the number of attenuation steps to be provided. This results in a more complex construction of switches and, further, a more complex configuration of the variable attenuator itself, making the size of the variable attenuator larger and its production cost higher.
To overcome the above problems, embodiments of the present invention provide a compact variable attenuator, a composite variable attenuator and mobile communication apparatus capable of variably controlling attenuation continuously in order to solve the problems described above.
One embodiment of the present invention provides a variable attenuator which comprises a first comb line consisting of first and second lines which are electromagnetically coupled, and a second comb line consisting of third and fourth lines which are electromagnetically coupled. First and second diodes are connected to the third and fourth lines of the second comb line, the first diode being connected between the third line and a ground with its anode connected to one end of the third line, the second diode being connected between the fourth line and a ground with its anode connected to one end of the fourth line, and the other ends of the first and third lines being connected and the other ends of the second and fourth lines, respectively, which are connected. A first terminal is connected to one end of the first line, and a second terminal is connected to one end of the second line. A first control terminal for turning the first diode on and off is connected to the junction of the other end of the first line and the other end of the third line and a second control terminal for turning the second diode on and off is connected to the junction of the other end of the second line and the other end of the fourth line.
Also, the variable attenuator of the present invention is characterized by being provided with a laminated ceramic substrate comprising a plurality of sheet layers made of ceramic, the ceramic substrate having strip-electrodes which form the first and second lines of the first comb line and the third and fourth lines of the second comb line, wherein the first and second diodes are mounted on the ceramic substrate.
A composite variable attenuator of the present invention is characterized by comprising a plurality of the above variable attenuators, wherein a plurality of variable attenuators are connected in cascade by connecting one end of the second line of a variable attenuator to one end of the first line of an adjacent variable attenuator.
Mobile communication apparatus of the present invention is characterized by using the above variable attenuator.
Also, it is characterized by using the above composite variable attenuator.
According to the variable attenuator of the present invention, since the first and second diodes are connected between one end of each of the third and fourth lines of the second comb line and the ground, it is possible to variably control the resistance of the first and second diodes by variably controlling the voltage being applied to the first and second diodes from the first and second control terminals. As a result, the loss in the first and second lines of the first comb line and that in the third and fourth lines of the second comb line can be variably controlled.
According to the composite variable attenuator of the present invention it is possible to expand the range of attenuation that can be variably controlled as a plurality of variable attenuators are connected in cascade.
According to the mobile communication apparatus of the present invention it is possible to achieve compact mobile communication apparatus, while maintaining receiving balance in the receiving system, because it uses a compact variable attenuator or compact composite variable attenuator.
Other features and advantages of the invention will be understood from the following description of embodiments thereof, with reference to the drawings, in which like references denote like elements and parts.